Competing Magnetic Interactions in Magnetoelectric YbMnO 3
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1161-I07-05
Competing Magnetic Interactions in Magnetoelectric YbMnO3 Shishir K. Ray, Ying Zou, Mark S. Williamsen, Somaditya Sen, Larry Buroker, and Prasenjit Guptasarma* Department of Physics, University of Wisconsin - Milwaukee, Milwaukee, WI 53211 ABSTRACT We present studies of magnetization and heat capacity of a single crystal of YbMnO3 in variable temperature and magnetic field, and clarify several new aspects of the magnetic fieldtemperature phase diagram. YbMnO3 is a rare-earth manganite oxide with hexagonal crystal symmetry in which two multiferroic ordered states – ferroelectricity and antiferromagnetism – coexist at low temperature. Single crystals of YbMnO3 were carefully grown from a Floating Zone (FZ) at low speed, then oriented and studied with the magnetic field oriented along the caxis. Magnetization and heat capacity measurement show features corresponding to long range anti-ferromagnetic (AFM) ordering of Mn3+, and the rare earth Yb3+. The ordering temperature of Mn3+ is independent of applied magnetic field up to 5T. However, contrary to previous reports in flux-grown crystals, we do not observe a complete suppression of Yb3+ order above 0.1T. Instead, we find that Yb3+ remains ordered at least up to 1 T, suggesting a revision of our current understanding of the ordering mechanism of the Mn-Yb and Yb –Yb sub-lattices in this hexagonal structure. INTRODUCTION Magnetoelectric rare earth manganite oxides (RMnO3) are a rare, relatively new, and fascinating class of materials in which ferroelectricity and (anti)ferromagnetism coexist within the same crystal phase [1,2]. The two ordered states are found to be coupled: applied magnetic field can change ferroelectric polarization, and an applied electric field can change magnetization, making them suitable for applications such as switching.[3] The hexagonal manganite YbMnO3 has received considerable attention in recent years due to the presence of ferroelectricity at room temperature (Tc = 973 K), low temperature magnetic properties and possible frustrated magnetic order (TNMn = 80 K, TNYb= 4K). The fascinating magnetoelectric properties at low temperature in hexagonal RMnO3 is believed be due to the rare earth atom being shifted from its central position and surrounded by triangular AFM order [4-9]. EXPERIMENT A single crystal of YbMnO3, grown by us from a floating zone, was used for all measurements reported here. Starting materials for the feed and seed rods for single crystal growth were fabricated by conventional solid state reaction of high-purity Yb2O3 and MnO2. Precursor oxides were mixed in stoichiometric proportions, ground in an agate ball mill, and reacted at a final temperature of 1200°C for 24 hrs in O2 after intermediate iterations of grinding and heating. The resulting single phase powder was packed inside silicone molds and pressed at 45 kPsi in an isostatic press, yielding high-density pressed rods 70mm in length and 6mm in diameter. The rods were subsequently sintered in Argon at 1200°C for 24hrs. Powder x-ray diffraction (PXRD) dat
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